The subject invention relates to measurements of nitrogen content in thin films formed on semiconductor wafers. In particular, complimentary approaches are disclosed for monitoring nitrogen content which include thermal wave technology as well as spectroscopic and ellipsometric measurements.
In the process of fabricating semiconductor devices, very thin films are used to form dielectric gates. Currently, silicon dioxide is the most common material used for the gate dielectric. With the push towards smaller devices, thinner gate dielectric layers are needed. Today, these layers are only 10 to 20 Angstroms thick. To obtain the necessary characteristics with these very thin layers, the industry is moving towards adding nitrogen to the silicon dioxide material.
The amount of nitrogen added to the silicon dioxide must be accurately controlled and therefore a precise method for measuring nitrogen concentration is required. Metrology efforts in the past have focused upon secondary ion mass spectrometry (SIMS) and X-Ray type measurements such as ESCA (Electron Spectroscopy for Chemical Analysis) or XPS (X-ray Photoemission Spectroscopy). Attempts have also been made to characterize nitrogen levels using either ellipsometry or spectroscopy.
It is believed that neither spectroscopy nor ellipsometry alone can provide sufficient information about nitrogen levels in a sample. Therefore, it would be desirable to develop one or more approaches for monitoring nitrogen levels that was fast, accurate and non-destructive.
In accordance with the subject invention, an approach has been developed which permits accurate evaluation of the nitrogen levels in an oxide layer. In this approach, two optical measurements of the semiconductor are made. The first measurement is based on a stable, narrowband ellipsometer. The information from the ellipsometer is useful for determining the thickness of the thin gate dielectric. This measurement is desired since an accurate determination of nitrogen levels based on an analysis of spectroscopic measurements also requires a very accurate knowledge of the layer thickness. A single wavelength, off-axis ellipsometer is one of the best tools for measuring the thickness of a very thin layer.
In accordance with the subject invention, a second measurement is made which is particularly sensitive to nitrogen concentration. This measurement is preferably a broadband multi-wavelength measurement. In initial experiments, it has been found that suitable information can be obtained from a reflectometry measurement, particularly concentrating in the UV wavelengths.
The measurements obtained from the narrowband ellipsometer and the reflectometer are used in combination to determine the thickness of the gate dielectric and the nitrogen concentration. More specifically, a theoretical model is set up which corresponds to the actual sample, including a substrate and at least the gate dielectric layer. The model includes various characteristics of the material, for example, thickness of the layer, index of refraction, and extinction coefficient. The model is typically seeded with initial parameters of the materials. Using the Fresnel equations, calculations are performed to determine expected measurement data if the modeled sample actually existed and was measured. This calculated data is then compared to the actual measured data. Differences between the calculated data and the actual measured data are then used to vary the expected characteristics of the sample of the model in an iterative process for determining the actual composition of the sample, including nitrogen levels.
The analysis of samples using a combination of a narrowband ellipsometer and another spectroscopic tool was described by assignee in PCT publication WO/9902970. This prior application described the benefits of using a narrowband ellipsometer to measure the thickness of a thin film or thin film stack and how that information can be combined with other measured data to characterize a multi-layer structure. This disclosure herein is directed to extending that measurement concept for evaluating nitrogen levels in a dielectric layer.
In initial experiments, the subject approach provided a highly accurate analysis. This approach is also relatively mathematically intensive. In certain on-line production situations, it is desirable to have a fast testing procedure for monitoring nitrogen levels in real time.
It has been discovered that another metrology approach, a thermal and/or plasma wave analysis, can be used to provide a faster, precise measurement. In these systems, an intensity modulated pump laser beam is focused on the sample surface for periodically exciting the sample. In the case of a semiconductor, thermal and plasma waves are generated in the sample which spread out from the pump beam spot. These waves reflect and scatter off various features and interact with various regions within the sample in a way which alters the flow of heat and/or plasma from the pump beam spot. (For convenience, the term xe2x80x9cthermal wavexe2x80x9d will be used for the remainder of the specification and claims to represent the wave like phenomenon associated with periodic excitation and includes both thermal and plasma waves.)
The presence of the thermal waves has a direct effect on the reflectivity at the surface of the sample. Features and regions below the sample surface which alter the passage of the thermal waves will therefore alter the optical reflective patterns at the surface of the sample. By monitoring the changes in reflectivity of the sample at the surface, information about characteristics below the surface can be investigated.
In one monitoring approach, a second laser is provided for generating a probe beam of radiation. This probe beam is focused collinearly with the pump beam and reflects off the sample. A photodetector is provided for monitoring the power of the reflected probe beam. The photodetector generates an output signal which is proportional to the reflected power of the probe beam and is therefore indicative of the varying optical reflectivity of the sample surface.
The output signal from the photodetector is filtered to isolate the changes which are synchronous with the pump beam modulation frequency. In the preferred embodiment, a lock-in detector is used to monitor the magnitude and phase of the periodic reflectivity signal. This output signal is conventionally referred to as the modulated optical reflectivity (MOR) of the sample.
The assignee herein markets a product which operates in accordance with these principals under the trademark Therma-Probe. This device incorporates technology described in the following U.S. Pat. Nos. 4,634,290; 4,636,088, 4,854,710 and 5,074,669. The latter patents are incorporated herein by reference.
It is also known that thermal wave effects can be measured with other forms of probes. In particular, the periodic excitation produces periodic movement (deformation) at the surface of the sample which can be monitored. Such techniques include interferometry as well as the measurement of the periodic angular deflection of a probe beam. Information about such systems can be found in U.S. Pat. Nos. 4,521,118; 5,522,510; 5,298,970; and PCT publications, WO 00/20841 and 00/68656, all of which are incorporated herein by reference. Such systems for monitoring the variations of a probe beam are within the scope of the subject invention.
In all of the thermal wave systems, information about both the amplitude and phase of the periodic signal generated from monitoring changes in the probe beam can be extracted. It has been found that these signals, and particularly the amplitude signal, vary with nitrogen concentration and thus can be used to monitor the nitration process. In practice, it would be difficult to use the thermal wave signal to provide an accurate value for the nitrogen concentration. Such accurate measurements can, however, be obtained from the above described combination of ellipsometric and broadband detection system which generates far more data and permits a more specific analysis to be made. In contrast, the thermal wave amplitude signal provides only a single value. Nonetheless, the sensitivity of the thermal waves to nitrogen concentrations is very high such that a thermal wave detection system can be used to precisely monitor a semiconductor fabrication process.
In the preferred embodiment, the thermal wave measurement technique is calibrated using the ellipsometer/broadband technique. More specifically, one or more samples can be measured using the more information rich ellipsometer/broadband measurement as well as the thermal wave technique. As noted above, the ellipsometer/broadband technique can provide accurate information about nitrogen content of the sample. This information can be correlated with the thermal wave measurements so that the thermal wave measurements will also give an accurate result for that type of sample. Thermal wave measurements can be made in real time and therefore can provide a simple evaluation of process parameters.
The sensitivity of the thermal wave technique to nitrogen concentration is present only before the wafer is annealed. During the annealing process, where the wafer is typically heated, the physical structure changes so that the thermal wave signal no longer varies with respect to nitrogen concentration. For this reason, the thermal wave signal is also ideal as an indication of proper annealing. More specifically, if the wafer has been fully annealed, it will produce the same thermal wave signal no matter what the nitrogen level. If the wafer is measured after the annealing process, the extent of the which the wafer was successfully annealed can be evaluated.
Further objects and advantages of the subject invention will become apparent with the following detailed description, taken in conjunction with the drawings in which: